In the forthcoming evolution of the mobile cellular standards like the Global System for Mobile Communication (GSM) and Wideband Code Division Multiple Access (WCDMA), new transmission techniques like Orthogonal Frequency Division Multiplexing (OFDM) are likely to occur. Furthermore, in order to have a smooth migration from the existing cellular systems to the new high-capacity high-data rate system in existing radio spectrum, a new system has to be able to utilize a bandwidth of varying size. A proposal for such a new flexible cellular system, called Third Generation Long Term Evolution (LTE), can be seen as an evolution of the 3G WCDMA standard. This system will use OFDM as the multiple access technique (called OFDMA) in the downlink and will be able to operate on bandwidths ranging from 1.25 MHz to 20 MHz. Furthermore, data rates up to and exceeding 100 Mb/s will be supported for the largest bandwidth. However, it is expected that LTE will be used not only for high rate services, but also for low rate services like voice. Since LTE is designed for Transmission Control Protocol/Internet Protocol (TCP/IP), Voice over IP (VoIP) will be the service that carries speech.
The LTE physical layer downlink transmission is based on OFDM. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, in which each so-called “resource element” corresponds to one OFDM subcarrier during one OFDM symbol interval.
As illustrated in FIG. 2, the downlink subcarriers in the frequency domain are grouped into resource blocks, where each resource block consists of twelve consecutive subcarriers for a duration of one 0.5 ms slot (7 OFDM symbols when normal cyclic prefixes are used (as illustrated) or 6 OFDM symbols when extended cyclic prefixes are used), corresponding to a nominal resource-block bandwidth of 180 kHz.
An important aspect of a terminal's operation is cell search. Cell search is the procedure by which the terminal finds a cell to which it can potentially connect. As part of the cell search procedure, the terminal obtains the identity of the cell and estimates the frame timing of the identified cell. The cell search procedure also provides estimates of parameters essential for reception of system information on a physical layer broadcast channel, containing the remaining parameters required for accessing the system.
To avoid complicated cell planning, the number of physical layer cell identities should be sufficiently large. For example, systems in accordance with the LTE standards support 504 different cell identities. These 504 different cell identities are divided into 168 groups of three identities each.
In order to reduce the cell-search complexity, cell search for LTE is typically done in several steps that make up a process that is similar to the three-step cell-search procedure of WCDMA. To assist the terminal in this procedure, LTE provides a primary synchronization signal and a secondary synchronization signal on the downlink. This is illustrated in FIG. 3, which illustrates the structure of the radio interface of an LTE system. The physical layer of an LTE system includes a generic radio frame 300 having a duration of 10 ms. FIG. 3 illustrates one such frame 300 for an LTE Frequency Division Duplex (FDD) system. Each frame has 20 slots (numbered 0 through 19), each slot having a duration of 0.5 ms which normally consists of seven OFDM symbols. A sub-frame is made up of two adjacent slots, and therefore has a duration of 1 ms, normally consisting of 14 OFDM symbols. The primary and secondary synchronization signals are specific sequences, inserted into the last two OFDM symbols in the first slot of each of subframes 0 and 5. In addition to the synchronization signals, part of the operation of the cell search procedure also exploits reference signals that are transmitted at known locations in the transmitted signal.
In the first step of the cell-search procedure, the mobile terminal uses the primary synchronization signal to find the timing of the 5 ms slots. Note that the primary synchronization signal is transmitted twice in each frame. One reason for this is to simplify handover of a call from, for example, a GSM system, to an LTE system. However, transmitting the primary synchronization signal twice per frame creates an ambiguity in that it is not possible to know whether the detected Primary Synchronization Signal is associated with slot #0 or slot #5 (see FIG. 3). Accordingly, at this point of the cell-search procedure, there is a 5 ms ambiguity regarding the frame timing.
In many cases, the timing in multiple cells is synchronized such that the frame start in neighboring cells coincides in time. One reason for this is to enable Multicast/Broadcast Single Frequency Network (MBSFN) operation. However, synchronous operation of neighboring cells also results in the transmission of the primary synchronization signals in the different cells occurring at the same time. Channel estimation based on the primary synchronization signal will therefore reflect the composite channel from all such cells if the same primary synchronization signal is used in those cells. For coherent demodulation of the second synchronization signal, which is different in different cells, an estimate of the channel from the cell of interest is required, not an estimate of the composite channel from all cells. Therefore, LTE systems support multiple sequences for the primary synchronization signals. To enable coherent reception of a particular cell's signals in a deployment with time-synchronized cells, neighboring cells are permitted to use different primary synchronization sequences to alleviate the channel estimation problem described above. If there is a one-to-one mapping between the primary synchronization signal used in a cell and the identity within a cell group, the identity within the cell group can also be determined in the first step.
In the next step, the terminal detects the cell group and determines the frame timing. This is done by observing pairs of slots in which the secondary synchronization signal is transmitted. To distinguish between Secondary Synchronization Signals located in subframe #0 and subframe #5, the Secondary Synchronization Signals are constructed in the form (s1, s2). If (s1, s2) is an allowable pair of sequences, where s1 and s2 represent the secondary synchronization signal in subframes #0 and #5, respectively, the reverse pair (s2, s1) is not a valid sequence pair. By exploiting this property, the terminal can resolve the 5 ms timing ambiguity that resulted from the first step in the cell search procedure, and determine the frame timing. Furthermore, as each combination (s1, s2) represents a particular one of the cell groups, the cell group identity is also obtained from the second cell search step.
Finally, the cell identity within the cell group needs to be determined (unless it was already obtained from the first step). One possibility for achieving this is to exploit the reference signals in the last step of the cell-search procedure. The reference signals are created as the product of a pseudo-random sequence and an orthogonal sequence. The pseudo-random reference signal sequence is given by the cell group identity, which is already known from the second step. Hence, only the orthogonal reference sequence remains to be determined. Therefore, as there is a one-to-one mapping between the orthogonal sequence and the identity within the cell group, the cell identity can simply be obtained by correlating the received signal with the product of the pseudo-random sequence identified in the second step and all possible orthogonal reference signals. The orthogonal reference signal producing the highest correlation is taken to be the one that was used to generate the reference signal sequence, and this orthogonal reference signal then indicates the cell group identity.
Once the cell search procedure is complete, the terminal receives the system information to obtain the remaining parameters (e.g., the transmission bandwidth used in the cell) necessary to communicate with this cell. This broadcast information is transmitted on the BCH transport channel.
Present plans call for the downlink BCH in LTE systems to be transmitted with a Transmission Time Interval (TTI) of 40 ms, which translates into one BCH transport block being transmitted once every 40 ms. FIG. 4 is a block diagram illustrating the physical layer logical elements involved in BCH transport block transmission. When a BCH transport block 401 is presented for transmission, it is processed by cyclic redundancy check (CRC) logic 403, which generates a CRC value that is appended to the BCH transport block 401 to enable error detection to be performed at the receiver. The output of the CRC logic 403 is then provided to a block 405 that performs channel coding (e.g., Forward Error correction Coding, or “FEC”), modulation, and other known processing. The resulting block is then supplied to a demultiplexor 407, which allocates the resulting bits for transmission in four 1 ms subframes 409 occurring within a 40 ms TI. In LTE systems, this transmission occurs within the first subframe (subframe #0) in each 10 ms radio frame. Other systems are conceivable in which the BCH transport block is transmitted in other subframes.
As a result of the initial cell search, the UE will have ascertained the frame (10 ms) timing from the primary and secondary synchronization signals. While this information tells the UE what the start times of frames are, the UE still does not know the frame number of any frames it can identify. Consequently, the UE does not know to what set of four frames a BCH transport block is mapped. For this to be known, the UE would have to know the 40 ins timing.
This problem is illustrated in the timing diagram of FIG. 5, in which are shown a series of 10 ins slots. Assume that it is desired to find the four frames associated with a BCH transmission identified as BCH transmission x. The BCH transmission occurring just prior to BCH transmission x is denoted BCH transmission x−1, and the one occurring just after BCH transmission x is denoted BCH transmission x+1.
In order to properly decode the BCH and (at the same time) find the 40 ms timing, the UE could make four different assumptions regarding the 40 ms timing (and hence, four different assumptions regarding where the start of the BCH transport block is). In the illustrated example, the first assumption 501 groups the last two frames of BCH transmissions x−1 with the first two frames of BCH transmission x. The second assumption 503 groups the last frame of BCH transmissions x−1 with the first three frames of BCH transmission x. The third assumption 505 groups all four frames of BCH transmissions x together, and hence represents the correct 40 ms timing. The fourth assumption 507 groups the last three frames of BCH transmissions x with the first frame of BCH transmission x.
An algorithm for finding the correct 40 ms timing and decoding the BCH in the process would involve carrying out the decoding of one or more of the four assumptions 501, 503, 505, 507, and determining which one was correct, based on the CRC. If a decoding is correct, the UE can assume that the corresponding timing is the correct 40 ms timing. In the specific case shown in FIG. 5, the third assumption produces the correct 40 ms timing.
A benefit of spreading the BCH transport block transmission out over four 1 ms subframes, is that, under good channel conditions (e.g. when the UE is close to the base station), the UE may only need to receive one, two, or three of the four subframes and still be able to decode the BCH properly. Under such circumstances, the UE does not need to engage its receiver during the time span of the four subframes, and has an opportunity for reducing its power consumption.
In order for this to be possible, the BCH transmission in each subframe should be self-decodable on a per subframe basis so that, under sufficiently good channel conditions, it should be possible to decode the BCH utilizing only the information within a single subframe.
FIG. 6 is a block diagram illustrating the physical layer logical elements involved in a straightforward way of achieving this. When a BCH transport block 601 is presented for transmission, it is processed by CRC logic 603, which generates a CRC value that is appended to the BCH transport block 601 to enable error detection to be performed at the receiver. The output of the CRC logic 603 is then provided to a block 605 that performs channel coding (e.g. “FEC”). (Modulation and other known processing are performed as well, but omitted from the figure for clarity). Here, the channel coding has a coding rate, denoted “R”, that enables the entire coded BCH to fit into a single subframe.
The resulting block is then supplied to copy logic 607, that creates a copy of the coded BCH bits, and allocates one copy into each of four 1 ms subframes 609 occurring within a 40 ms TTI. In LTE systems, this transmission occurs within the first subframe (subframe #0) in each 10 ms radio frame. Other systems are conceivable in which the BCH transport block is transmitted in other subframes.
Under good channel conditions, the coding will enable the UE to correctly decode the BCH transport block using the information (bits) conveyed by only a single one of the four subframes. When worse channel conditions prevail, the UE needs more energy for correct decoding. The UE can satisfy this requirement by soft combining (e.g. simply adding together) the signals received from multiple subframes (i.e., from two to four subframes) before decoding.
Another way of transmitting the BCH information in such a way that it is possible to correctly decode it using bits received from fewer than all four subframes is illustrated in FIG. 7. When a BCH transport block 701 is presented for transmission, it is processed by CRC logic 703, which generates a CRC value that is appended to the BCH transport block 701 to enable error detection to be performed at the receiver. The output of the CRC logic 703 is then provided to a block 705 that performs channel coding (e.g. “FEC”). (Modulation and other known processing are performed as well, but omitted from the figure for clarity). This arrangement differs from that depicted in FIG. 6 in that a lower channel coding rate is used (e.g., four times lower, represented by “R/4”).
The resulting bits are then supplied to demultiplexor logic 707, that distributes the coded bits over the full set of four 1 ms subframes 709 occurring within a 40 ms TTI. In LTE systems, this transmission occurs within the first subframe (subframe #0) in each 10 ms radio frame. Other systems are conceivable in which the BCH transport block is transmitted in other subframes.
In this case, the arrangement relies on the channel code's capability to correct loss of data. In essence, using only a single subframe for the BCH decoding would correspond to a loss of at least 75% of the coded bits. A sufficiently low rate code Rlow can, in such a case, still provide correct decoding of the BCH transport block if the channel conditions are sufficiently good.
It is noted that an equivalent way of interpreting arrangements exemplified by that depicted in FIG. 6 is to assume that the lower rate channel code of FIG. 7 consists of a higher rate code followed by four times repetition. Such an arrangement is depicted in FIG. 8. When a BCH transport block 801 is presented for transmission, it is processed by CRC logic 803, which generates a CRC value that is appended to the BCH transport block 801 to enable error detection to be performed at the receiver. The output of the CRC logic 803 is then provided to a block 805 that performs channel coding (e.g., “FEC”). (Modulation and other known processing are performed as well, but omitted from the figure for clarity). The coding rate here is R (i.e., the rate that enables the entire coded BCH to fit into a single subframe).
The coded bits are then supplied to copy logic 807 that produces four copies of the coded BCH bits. The resulting bits supplied by the copy logic 807 are then supplied to demultiplexor logic 809, that distributes the coded bits over the full set of four 1 ms subframes 811 occurring within a 40 ms TTI. In LTE systems, this transmission occurs within the first subframe (subframe #0) in each 10 ms radio frame. Other systems are conceivable in which the BCH transport block is transmitted in other subframes.
It can be seen that allowing the BCH information contained in any one of the four subframes to be self-decodable (i.e. decodable without requiring BCH information from any of the remaining three subframes) can achieve significant efficiencies under good channel conditions. However, a problem with the self-decodability techniques illustrated in any of FIGS. 6, 7, and 8 is that the possibility of ascertaining the 40 ms timing can no longer be guaranteed. In essence, if the UE decodes the BCH using only one of the BCH subframes illustrated, for example, in FIG. 6, it can get a correct decoding (correct CRC) regardless of what subframe is used for decoding. Thus the UE will not be able to know which of the four possible subframes was used for the decoding and will thus not ascertain the 40 ms timing.
It should be noted that this is only a problem when the UE does not yet know the full timing of the cell. However, this does not render the problem insignificant because this is the case when, for example, the UE is making an initial access of the cell, or at the time of handover between cells that are not tightly time synchronized with each other.
It is therefore desired to provide methods and apparatuses that enable a UE (or other terminal in a mobile communications system) to be capable of correctly decoding BCH information based on fewer than all of the transmissions of that information (e.g., only one of four transmissions of that information), while still being capable of ascertaining the timing of those transmissions.